24,027 reputation
155128
bio website lightandmatter.com
location Fullerton, California
age
visits member for 3 years, 3 months
seen 2 hours ago

I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.


1h
comment Why GPS is at LEO?
@JerrySchirmer: That's not quite right. The usual goal is to locate yourself on a map, i.e., horizontally, and for that you get the best accuracy when the satellites are close to the horizon.
2h
comment Sound diffraction through a single slit
I wasn't the person who flagged it as low quality, but the original answer was only one sentence.
2h
comment Sound diffraction through a single slit
I don't think this is right, for the reasons given in my answer. Note that I gave three separate mechanisms for the diffraction to behave differently.
2h
comment Sound diffraction through a single slit
@Floris: You could also worry about the dielectric constant of the material, etc. You can't just assume a material with no electromagnetic properties -- such a material would be perfectly transparent.
2h
comment Sound diffraction through a single slit
related: physics.stackexchange.com/questions/5886/… physics.stackexchange.com/questions/141562/…
3h
comment Recommended books for undergraduate electrodynamics
@ChrisWhite: No way. Jackson has much more depth and breadth than Purcell. I learned E&M from Purcell as an undergrad and had Jackson in grad school, and there was a ton I learned from Jackson.
7h
comment For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
The force equations you provided are wrong. He's applying Newton's second law to the pulley, which is perfectly legitimate.
7h
comment For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
I see. I edited the question a little and retracted my close vote.
8h
comment For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
Clearly, since there is a net upward force, the pulley itself will accelerate upwards. Huh? Why doesn't the whole apparatus just drop?
8h
comment If something is not moving in space, is it moving on the time axis at the speed of light?
This is why people say "if something is not moving in space, then it is moving on the time axis at the speed of light." "People" is the popularizer Brian Greene. Relativists in general don't talk this way.
8h
comment For a massless pulley moving upwards with acceleration, is the upward force equal to the downward force?
What do you mean by "not attached to a ceiling?" Are you saying that the whole setup is just free-falling straight down? If it's not attached to a ceiling, then what object is exerting the force F on the pulley?
22h
comment What is logical way to calculate percentage error?
I disagree on both counts. There is no particular reason to prefer division by A or by B in general, and it is not true that we don't care about the sign. As an example where you would certainly not want to divide by $B$, there could be a case where you're testing a theoretical prediction that $B=0$.
22h
comment What is logical way to calculate percentage error?
Percent error is almost never of interest, so the real answer is that working scientists would never worry about this issue. If you're testing an experiment against theory, there's no way to know whether a 0.03% difference is consistent with the theory or inconsistent with it, because it depends on how much error would have been expected due to the inherent precision of the technique. In real science we would say we measured A=____$\pm$____, and compared with the predicted value B=____ this was off by, e.g., 5.7 std dev, which is highly statistically significant, so the theory is disproved.
23h
comment Liouville's theorem and preservation of topology
@StevenMathey: Then you can start by asking where do the points that are exactly on the line go? Can't they just stay where they are, i.e., this is an equilibrium? Moreover, two points that are separated by this line can be chosen as close to each other as you want. If this line existed, they would end up far away from each other whatever their original distance. Isn't this exactly what we expect if it's an unstable equilibrium?
1d
comment Liouville's theorem and preservation of topology
Let's say my original connected subset R is a set of initial conditions for a pencil very nearly balanced on its tip. Doesn't this split into two disconnected parts, one in which the pencil falls to the right and one in which it falls to the left?
1d
comment The classical hydrogen atom
We expect accelerations at the atomic level to be huge, simply because subatomic particles are traveling very fast (the electron in hydrogen at ~1% of the speed of light), and are confined to very small regions of space.
1d
comment The classical hydrogen atom
related: physics.stackexchange.com/q/17651
1d
comment Recommended books for undergraduate electrodynamics
The first edition of Purcell was written under an NSF grant and is therefore legally and freely available online from sites such as Library Genesis. If you have the coin, the third edition is nicer (contains a lot more applications, and uses MKS). It's theoretically a freshman book, not upper division, but in reality it's about the same level as Griffiths. Greatest E&M book ever.
1d
comment Underdetermined forces in a statics problem
The wheels argument would simply seem to be an argument that any time there is equilibrium, there can be no static friction; you haven't used anything specific to the present problem. But we know that static friction can exist when there is equilibrium. Changing a solid surface to a wheeled surface simply changes the behavior of the surface.
1d
comment Underdetermined forces in a statics problem
It seems reasonable to imagine that the amount of friction depends on deformation. However, I don't see any reason to infer that when the box is perfectly rigid, the friction must vanish, and I don't buy the argument in the 2nd paragraph about wheels. I think the assumption of perfect rigidity simply makes the problem underdetermined -- it doesn't make F=0. Solutions with $F\ne 0$ exist for any finite rigidity of the box, so I don't see how one can argue otherwise in the limit of infinite rigidity.